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Improving the Jason-1 Ground Retracking to Better Account for Attitude Effects

机译：改善Jason-1地面重新跟踪以更好地说明态度影响

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摘要

After two years of verification and validation activities of the Jason-1 altimeter data, it appears that all the mission specifications are completely fulfilled. Performances of all instruments embarked onboard the platform meet all the requirements of the mission. However, the star tracker system has shown some occasional abnormal behavior leading to mispointing angles out of the range of Jason-1 system specification which states that the altimeter antenna shall be pointed to the nadir direction with an accuracy below 0.2 degree (3 sigma). This article discusses the platform attitude angle and its consequences on the altimetric estimates. We propose improvements of the Jason-1 retracking process to better account for attitude effects. The first star tracker anomalies for the Jason-1 mission were detected in April 2002. The Poseidon-2 algorithms were specified assuming an antenna off-nadir angle smaller than 0.3 degree. For higher values, the current method to estimate the ocean parameters is known to be inaccurate. Thus, the algorithm has to be reviewed, and more specifically, the present altimeter echo model has to be modified to meet the desired instrument performance. Therefore, we derive a second order analytical model of the altimeter echo to take into account attitude angles up to 0.8 degree, and consequently, we adapt the retracking algorithm. This new model is tested on theoretical simulated data using a maximum likelihood estimation. Biases and noise performance characteristics are computed for the different estimated parameters. They are compared to the ones obtained with the current algorithm. This new method provides highly improved estimations for high attitude angles. It is statistically validated on real data by applying it on several cycles of Poseidon-2 raw measurements. The results are found to be consistent with those obtained from simulations. They also fully agree with the TOPEX estimates when flying along the same ground track. Finally, the estimates are also in agreement with the ones available in the current I/GDE (Intermediate Geophysical Data Record) products when mispointing lies in the mission specifications.
机译：经过两年的Jason-1高度计数据的验证和确认活动，看来所有任务规范都已完全实现。平台上搭载的所有仪器的性能均能满足任务的所有要求。但是，恒星跟踪器系统偶尔会出现一些异常行为，从而导致角度错误指向超出Jason-1系统规范的范围，该规范指出高度计天线应指向天底方向，精度应低于0.2度（3 sigma）。本文讨论了平台姿态角及其对高度估计的影响。我们建议对Jason-1重新跟踪过程进行改进，以更好地说明态度影响。在2002年4月检测到了Jason-1任务的第一个恒星跟踪仪异常。指定Poseidon-2算法时假定天线的离天底角小于0.3度。对于更高的值，目前估算海洋参数的方法是不准确的。因此，必须对算法进行审查，更具体地说，必须对当前的高度计回波模型进行修改以满足所需的仪器性能。因此，我们得出了高度计回波的二阶分析模型，其中考虑了高达0.8度的姿态角，因此，我们采用了重新跟踪算法。使用最大似然估计在理论模拟数据上测试了该新模型。针对不同的估计参数计算偏差和噪声性能特征。将它们与使用当前算法获得的结果进行比较。这种新方法为高姿态角提供了高度改进的估计。通过将其应用于多个Poseidon-2原始测量周期，可以对实际数据进行统计验证。发现结果与从模拟获得的结果一致。他们还完全同意TOPEX在同一地面轨道上飞行时的估计。最后，当错误指出任务规格时，估算值也与当前I / GDE（中间地球物理数据记录）产品中的估算值相符。

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